Zinc-halide batteries were developed as devices for storing electrical energy. Traditional zinc-halide batteries (e.g., zinc-bromine batteries) employed bipolar electrodes disposed in a static, i.e., non-flowing, zinc-bromide aqueous solution. The process of charging and discharging electrical current in a zinc-halide battery is generally achieved through a reaction of redox couples like Zn2+/Zn(s) and X−/X2 in zinc halide electrolyte. When the battery is charged with electrical current, the following chemical reactions occur:Zn2++2e−→Zn2X−→X2→2e−,wherein X is a halogen (e.g., Cl, Br, or I). Conversely, when the battery discharges electrical current, the following chemical reactions occur:Zn→Zn2++2e−X2+2e−→2X−.
These zinc-halide storage batteries were formed in a bipolar electrochemical cell stack, wherein each electrode comprises two poles, such that the anodic reaction occurs on one side of the electrode, and the cathodic reaction occurs on the opposite side of the same electrode. In this vein, bipolar electrodes were often configured as plates, and the cell stack was assembled to form a prismatic geometry. During charging and discharging of the bipolar battery, the electrode plates function as conductors for adjacent cells, i.e., each electrode plate serves as the anode for one cell and the cathode for the adjacent cell. In this prismatic battery geometry, the entire surface area of the electrode plate that separates adjacent electrochemical cells transfers current from cell to cell.
Accordingly, when a traditional bipolar zinc-halide battery charges, zinc metal electrolytically plates on the anode side of the bipolar electrode plate while molecular halogen species form at the cathode side of the electrode plate. And, when the battery discharges, the plated zinc metal is oxidized to free electrons that are conducted through the electrode plate and reduce the molecular halogen species to generate halide anions.
However, battery performance of traditional zinc-halide batteries is severely limited due to uneven zinc plating on the bipolar electrode during charging. Uneven or irregular zinc plating on the bipolar electrode generates zinc dendrites in the battery and reduces battery capacity and cycle life. Moreover, uneven zinc plating creates heterogeneity in battery discharge currents that negatively affects battery performance as a storage device for electrical energy.